In recent years, rapid progress of an LCD (liquid crystal display) is remarkable in a field of an FPD (flat panel display). The liquid crystal display has been in the spotlight as a core element of not only in a field of a small and medium size display which has been typically used, but also in fields of a small mobile display, a large size television, and an outdoor screen.
However, unlike a self-emitting display such as a CRT (cathode ray tube), a PDP (plasma display panel), and an LED (light emitting diode) display, the liquid crystal display is a non-self emitting display or a non-emissive display which does not emit light by itself so that the liquid crystal display does not spontaneously emit light. Therefore, the liquid crystal display requires a separate light source in order to output an image.
A BLU (back light unit) is a type of a light source device which supplies light to a rear side of a screen of a liquid crystal display. The BLU is a core component which not only directly affects an image quality such as brightness of an image, a color reproduction degree, a viewing angle, a contrast ratio, or readability, power consumption, or a lifespan of a product, but also accounts for approximately 20 to 50% of an entire unit price of the liquid crystal display.
The back light unit is mainly classified into a direct-lit and an edge-lit depending on an arrangement pattern of a light source. In the direct-lit back light unit, a light source is disposed immediately at a rear side of a screen to emit light toward a liquid crystal panel but in the edge-lit back light unit, a light source is disposed at a periphery of the screen to emit light to the side and a light guide plate guides the light to be irradiated toward the liquid crystal panel. When the direct-lit back light unit and the edge-lit back light unit are compared, generally, the edge-lit back light unit is advantageous in terms of a thickness or a cost of the product but the direct-lit back light unit is advantageous in terms of brightness, a contrast ratio, uniformity of a screen, and image reproducibility.
As described above, the edge-lit back light unit is excellent in terms of a thickness of the product but an image quality of the edge-lit back light unit is lower than the direct-lit back light unit. The image quality of the edge-lit back light unit mainly depends on a light guide plate which supplies light, which is incident from the light source to the side, to a liquid crystal display panel in the form of a surface light source. Specifically, when the thickness of the light guide plate and the number of light sources are reduced in order to reduce a weight and a thickness of the display device, a dark portion is generated between a point light source and a point light source and an area of a light entrance portion which the light reaches is reduced so that light entrance efficiency is lowered. As a result, an amount of light which is supplied to the display panel is reduced so that brightness is lowered. Therefore, a light guide plate through which brightness is less lowered and light output is uniformly distributed is strongly demanded.